Hematopoietic stem cells (HSCs) can respond to microenvironmental cues that modulate cell proliferation, differentiation, survival, self-renewal, and migration. These responses are critical for the HSC to contribute to hematopoiesis long-term during steady-state and during stress. Defects in these responses can lead to hematologic disease. The most durable and complete cure for many diseases would be stem cell replacement, however stem cell transplant is currently not warranted for many benign disorders. Understanding the basic biology of HSCs may lead to novel approaches for stem cell transplant and for promoting endogenous hematopoietic recovery following myeloablation. These goals may have significant clinical benefit. Furthermore, the same mechanisms of HSC regulation may be involved in leukemic stem cell biology. Our research during the first cycle of this R01 was focused on defining the role of JAK/STAT signaling in normal HSC biology. This work uncovered roles for STATS activation in growth, long-term competitive repopulating activity, and self-renewal of HSCs. We showed that deficiencies in HSCs were almost identical to c-MpI knockout mice but deficiencies in multipotent progenitor differentiation were considerably greater, illustrating a broad requirement for STATS in hematopoiesis. Despite these major deficiencies, STATS knockout mice were relatively normal in functions such as survival and migration. These mice were able to engraft lethally-irradiated hosts and non-irradiated W/WV hosts with primarily T cell reconstitution defects. However, we were surprised to find that a Balb/c derived modifier locus that includes the Gab2 gene modulated the steady-state engraftment ability as measured in W/WV hosts. Since STATS cooperation with Gab2 and PI3-kinase signaling has been described by others, we aim to test the hypothesis that a significant proportion of STATS-mediated activity in HSCs can be mediated in concert with Gab2, with an emphasis on survival and migration responses to cytokines. Furthermore, we found that STATS knockout mice expressed low levels of a functional N-terminal truncated isoform of STATS (STAT5AN) that might compensate for HSC deficiencies that were not uncovered previously. Therefore we aim to do the to the following; 1) Determine whether STAT5AN is compensating for survival and migration functions of STATS in HSCs by studies using a new "true null" STATS mouse; 2) Use genetic approaches in mice to determine whether Gab2 can cooperate with STATS for normal HSC functions; 3) Determine the role of STATS and Gab2 in myeloproliferative disease associated with constitutive activation of STATS. This work is expected to significantly advance our understanding of the pivotal role of STATS signaling in normal and leukemic stem cells and potentially uncover new therapeutic strategies.